Cholera remains a significant public health problem in Vietnam. During the late 1980s, the National Institute of Hygiene and Epidemiology of Vietnam developed a killed oral cholera vaccine modelled on a Swedish vaccine tested by Branch investigators in Bangladesh during the mid-1980s. A field trial of this vaccine in Hue, which enrolled 100,000 subjects, found the vaccine to confer about 60% protection against El Tor cholera, with similar efficacy in young children and in older persons. This earlier vaccine has now been augmented to include coverage against the newly recognized 0139 serogroup of V. cholerae. A pretest of the safety and immunogenicity of this vaccine was recently conducted in adults in Hanoi. During the next 9 months, a similar pretest will be conducted in Hanoi children, and a large-scale field trial of the vaccine will be initiated in the coastal city of Nha Trang next March. This trial will enroll about 200,000 persons, and will assess the clinical effectiveness of the vaccine over two years following vaccination.